1. Field of the Invention
The present invention relates generally to growth of crystalline materials and, in particular, to Czochralski growth of doped silicon.
2. Description of the Related Art
The technology of growing crystalline silicon ingots according to the Czochralski (CZ) method has been extensively developed over many decades to supply silicon wafers to the integrated circuit (IC) industry and the photovoltaic (PV) solar industry. In the Czochralski method, a crucible composed of a refractory material such as fused quartz and containing silicon is heated to the melting point of silicon at around 1416° C. to produce a silicon melt in the crucible. A seed of crystalline silicon of predetermined crystallographic orientation is lowered to barely contact the melt. Molten silicon freezes on the seed with the same orientation. The seed is slowly withdrawn from the melt and the process continues to grow an ingot of monocrystalline silicon having a diameter of perhaps 200 or 300 mm and a length of a meter or more. After drawing, the ingot is sliced to produce silicon wafers, which are then further processed to produce IC's or PV solar cells.
The IC industry predominantly relies on the batch Czochralski process in which a crucible is initially charged with silicon shards, chunks, or pellets of very high purity and then heated to the silicon melting point. Typically, after the growth of one silicon ingot, the crucible is discarded and replaced by a fresh crucible. Although the PV solar industry has utilized batch CZ silicon wafers, continuous Czochralski (CCZ), more accurately called semi-continuous CZ, has been proposed in which during the growth of an ingot the crucible is continuously or at least intermittently supplied with silicon feedstock such that the level of silicon melt in the crucible remains substantially constant. After a normal sized ingot has been drawn from the crucible, another ingot can be drawn by using a new seed or reusing the previous seed. The process may be repeated for a substantial number of ingots determined by factors such as accumulation of impurities or degradation of the crucible. The CCZ process reduces the cost of crucibles, increases manufacturing throughput, and in some respects simplifies the thermal control along the length of the ingot. However, CCZ also requires resupply directly into the hot crucible while it is growing crystalline silicon.
Wafers, including wafers destined for solar applications, are preferentially grown doped to a desired conductivity type and dopant concentration, usually simply measured by resistivity. The fabrication of doped silicon wafers of one conductivity types allows the formation of a p-n junction by the diffusion or implantation of a layer of dopant of the other conductivity type. Semiconductor dopants are well known, most particularly boron (B) for p-type and phosphorous (P) for n-type although gallium (Ga) offers many advantages as a p-type dopant for solar cells.
Doped silicon can be grown according to the CZ method by including in the melt a required level of the dopant, and the dopant is incorporated from the melt into the silicon ingot along with the silicon. Doping concentrations relative to silicon are typically much less than 1 ppma (part per million atomic). However, the process is complicated by the segregation effect according to which, if the concentration of a dopant in the melt is C, the dopant concentration in the solidified ingot is kC, where k is the segregation coefficient, which is usually less than one and often much less than one. Segregation coefficients for several dopants in silicon are given in TABLE 1.
TABLE 1ElementkB0.8P0.35Ga0.008In0.0004As0.3Al0.002
For batch CZ, the dopant can often be charged into the cold crucible together with the solid silicon feedstock and the two melted together as the crucible is heated to the silicon melting point. However, the segregation effect causes a larger fraction of silicon than of the dopant in the melt to be supplied to the growing ingot. Although the dopant concentration in the pre-charge can be adjusted to produce the desired doping in the ingot, the segregation effects causes the concentration of the dopant to gradually increase in the melt as the crystal grows and the melt is depleted. As a result, the later portions of the ingot have a higher dopant concentration (lower resistivity) than the initial portions. In batch CZ, gallium doping can vary by a factor of ten and boron doping can vary by about 30%. Such axial variation (i.e., along the length of the ingot) is not desired since the resistivity of wafers then depends upon their position within the ingot. Because solar panel performance is negatively affected by cell non-uniformity, cell manufactures include costly wafer sorting steps in their fabrication lines. Uniformity of the wafer supply can help in eliminating these steps. Another application that would benefit from this invention is the making of heavily doped rods of uniform resistivity that can be sectioned and mixed as solid with feedstock in subsequent CZ manufacturing processes to make rods of relatively lower dopant concentration and higher resistivity.
In continuous CZ, an amount of dopant which, when melted, provides a desired concentration in the liquid may be supplied together with the fresh silicon feedstock in order to maintain a constant dopant concentration in the melt and thus assure a constant axial resistivity within each ingot and a constant resistivity among ingots. However, the continuous resupply of dopant in elemental form in CCZ presents difficulties arising from the lower melting points of most dopants relative to silicon and the need to separately meter controllable amounts of dopant into the hot crucible. Gallium presents particular difficulties because of its melting point of about 30° C. and its very low segregation coefficient. Continuous doping with elemental dopants, such as boron and phosphorous that are typically in powder form, is also quite difficult in practice.
Accordingly, better and more economical methods and apparatus are desired for the Czochralski growth of ingots with axially uniform resistivity.